Dust evolution in protoplanetary disks around Herbig Ae/Be stars - The Spitzer view

摘要

In this paper we present mid-infrared spectra of a comprehensive set ofHerbig Ae/Be stars observed with the Spitzer Space Telescope. Thesignal-to-noise ratio of these spectra is very high, ranging between about ahundred and several hundreds. During the analysis of these data we tested thevalidity of standard protoplanetary dust models and studied grain growth andcrystal formation. On the basis of the analyzed spectra, the major constituentsof protoplanetary dust around Herbig Ae/Be stars are amorphous silicates witholivine and pyroxene stoichiometry, crystalline forsterite and enstatite andsilica. No other solid state features, indicating other abundant dust species,are present in the Spitzer spectra. Deviations of the synthetic spectra fromthe observations are most likely related to grain shape effects anduncertainties in the iron content of the dust grains. Our analysis revealedthat larger grains are more abundant in the disk atmosphere of flatter disksthan in that of flared disks, indicating that grain growth and sedimentationdecrease the disk flaring. We did not find, however, correlations between thevalue of crystallinity and any of the investigated system parameters. Ouranalysis shows that enstatite is more concentrated toward the warm inner diskthan forsterite, in contrast to predictions of equilibrium condensation models.None of the three crystal formation mechanisms proposed so far can aloneexplain all our findings. It is very likely that all three play at least somerole in the formation of crystalline silicates.